The objectives of this research project are to investigate possible developmental roles of neurotransmitters (the monoamines) in early phases of neuroembryogenesis (neural tube closure and the time of origin of their prospective target cells) and the modulation of this ontogenetic regulation by hormones (the steroids). Various methods will be employed in these studies of early neurogenesis in the rat and chick, including light and electron microscopic autoradiography, fluorescence histochemistry, immunocytochemistry and radioenzymatic microassays. The effects of monoamine-depleting drugs, stress, diet, and the hormonal integrity of the pregnant rat on fetal brain development will also be explored in the same experimental context. The results of these studies bear direct relevance to the problems of neural tube defects and mechanisms underlying the initial phase of circuitry construction in the developing brain.